1. Field of the Invention
The present invention relates generally to signal processing. More particularly, the invention relates to circuits and methods for analog-to-digital (A/D) conversion using a threshold comparator which includes superconducting devices.
2. State of the Art
Known signal processors include comparators for processing analog signals into digital signals. To address the need for faster, more accurate signal processing, A/D converters have been proposed which include comparator circuits formed with superconducting elements, such as Josephson junction elements (i.e., Josephson junctions).
Josephson junctions are described in a document entitled "Superconducting electronics", Physics Today, February 1981 by Donald G. McDonald, the disclosure of which is hereby incorporated by reference in its entirety. As described therein, Josephson junctions are devices which exploit the concept of magnetic flux quantization. Generally speaking, magnetic flux quantization refers to the ability of superconducting loops, or rings, to trap the magnetic field of a permanently circulating supercurrent.
Josephson junctions are typically formed from two thin films of superconducting metals separated by a thin insulating layer. An electrical current is conducted across the two thin films with zero voltage drop provided the current is below a predetermined maximum level (e.g., approximately lmA) referred to as the "critical current".
Comparator circuits formed with Josephson junctions generally fall into one of two classes: those that directly detect injected currents, and those that detect magnetic flux. Examples of the former include a single Josephson junction sampling gate and a pair of Josephson junctions operated in a differential mode. Examples of the latter include Superconducting Quantum Interference Devices (i.e., SQUIDs) and Quantum Flux Parametrons (i.e., QFPs).
A SQUID is typically formed as a superconducting loop interrupted by two Josephson junctions. Maximum current through the superconducting loop occurs when either junction reaches its critical current. Because the maximum current is a periodic function of magnetic flux through the SQUID, these devices provide a finely graded measuring scale for magnetic flux detection. The frequency with which the maximum current is detected (i.e., the frequency with which a voltage drop is detected across at least one Josephson junction) represents a measure of magnetic flux through the SQUID.
A document entitled "A Single-Chip SQUID Magnetometer", IEEE Transactions On Electron Devices, Vol. 35, No. 12, December 1988 by Norio Fujimaki et al. further describes a SQUID magnetometer for quantizing and permanently storing flux in a superconducting loop. The SQUID magnetometer includes a SQUID sensor which changes from a zero-voltage state to a finite voltage state (e.g., approximately lmV) when an AC bias current pulse crosses a threshold value. The threshold value is a function of magnetic flux coupled to the SQUID and depends upon characteristics of the SQUID sensor (i.e., the inductance, the Josephson junction critical currents and the location of the bias current injection point).
A known QFP used for polarity discrimination of an input signal is disclosed in U.S. Pat. No. 4,916,335 (Goto et al), the disclosure of which is hereby incorporated by reference in it entirety. See also "Basic Operations of the Quantum Flux Parametron", Harada, Y. et al., MAG-23, September 1987, p. 3801. Like the SQUID, a QFP typically includes a superconducting loop interrupted by two Josephson junctions for amplifying weak magnetic flux. As the center node inductance of a QFP is increased, its functional behavior approaches that of a SQUID.
Single pass Josephson junction A/D converters have been designed using SQUIDs with two and three Josephson junctions (see, for example, "Superconducting A/D Converter Using Latching Comparators", C. A. Hamilton et al., IEEE Tran. Magn., vol MAG-21, pp. 197-199 and "Analog Signal Processing With Josephson Junctions: Analog to Digital Conversion", Spargo et al., Extended Abstracts of the 1987 International Superconductivity Electronics Conference, Aug. 28, 1987, Tokyo, pp. 319-334). Single pass A/D converters are attractive because of their increased conversion speed relative to known multiple pass A/D converters (e.g., successive approximation and parallel feed-forward A/D converters). However, implementation of a fully parallel A/D converter, which requires 2.sup.n -1 comparators for n bits of resolution has not been practical in superconductive circuits because of the tight tolerance required in the components. (See, for example, G. S. Lee and D. A. Petersen, Superconductive A/D Converters, Proceedings of the IEEE, 77(8) August 1989, P. 1264).
The foregoing limitations can be traced to a general problem with implementing high resolution converters; i.e., an inability to produce devices with uniform characteristics. For example, a significant barrier to making superconductive A/D converters has been the difficulty in matching circuit elements, such as resistors and Josephson junctions.